A wealth of cell cycle research supports the idea that the cellular transcription factor DRTF1/E2F plays a pivotal role in coordinating early cell cycle events by integrating transcription with cell cycle progression (Nevins, 1992; La Thangue, 1994). For example, the negative effects on cellular proliferation exerted by the retinoblastoma tumour suppressor gene product (pRb) appear to be mediated in part through the ability of pRb to regulate the transcriptional activity of DRTF1/E2F (Heibert et al., 1992; Zamanian and La Thangue, 1992). Since many target genes encode proteins which contribute to cell cycle progression, pRb may influence proliferation through a regulation of DRTF1/E2F. The importance of this pathway in cell cycle control is underscored by the fact that natural mutations in Rb, which frequently occur in human tumour cells, encode proteins which fail to bind to DRTF1/E2F and that the oncogene products of certain tumour viruses, such as adenovirus Ela, human papilloma virus E7 and SV40 large T antigen bind pRb, an interaction which correlates with a potential to mediate cellular transformation (Nevins, 1992; La Thangue, 1994).
Other members of the pRb family (known generically as `pocket proteins`) interact with DRTF1/E2F and regulate its transcriptional activity. Two other members of the family, plO7 and p130, associate with DRTF1/E2F in temporally distinct fashions during cell cycle progression, p130 predominantly during G0 and p107 in late G1 and into S phase (Shirodkar et al., 1992; Cobrinik et al., 1993). Like pRb, the physical association of p107 leads to transcriptional inactivation (Schwarz et al., 1993; Zamanian and La Thangue, 1993), and p107 can negatively regulate cell cycle progression (Zhu et al., 1993) but in contrast to Rb is not known to be mutated in tumour cells.
Several other molecules involved in regulating the cell cycle interact with DRTF1/E2F. Either cyclin A or E, together with the catalytic kinase subunit cdk2, can stably interact in a fashion which is dependent upon the presence of p107 or p130; again these events are under temporal control during the cell cycle (Lees et al., 1992; Cobrinik et al., 1993; Li et al., 1993). The physiological role of the cyclin A/cdk2 and cyclin E/cdk2 kinase in this context is unknown. In contrast, there is persuasive evidence that cyclins A and E influence the growth regulating capacity of pRb (Hinds et al., 1992). An alternative type of interaction can occur between a cyclin A-dependent kinase and DRTF1/E2F (Dynlacht et al., 1994; Krek et al., 1994). The biochemical consequence of this interaction is reduced DNA binding activity which is believed to be involved with the physiological control of transcription at later times during the cell cycle.
Progress has been made in understanding the molecular composition of DRTF1/E2F. Specifically, generic DRTF1/E2F DNA binding activity arises when members of two distinct families of proteins interact as DP/E2F heterodimers (Lam and La Thangue, 1994), the prototype molecules of each family being E2F-1 (Helin et al., 1992; Shan et al., 1992; Kaelin et al., 1992) and DP-1 (Girling et al., 1993). Heterodimerization between DP and E2F proteins (Bandara et al., 1993; Helin et al., 1993; Krek et al., 1993; Girling et al., 1994) allows a variety of combinatorial interactions to generate an array heterodimers.
Information on the properties of E2F family members suggests that they perform a physiological role in dictating the nature of the pocket protein which physically interacts with the heterodimer. From the E2F family members characterised, it is believed that E2F-1, -2 and -3 interact with pRb (Ivey-Hoyle et al., 1993; Lees et al., 1993), E2F-4 with p107 (Beijersbergen et al., 1994; Ginsberg et al., 1994) and E2F-5 with p130 (Buck et al., 1995). An extreme C-terminal region in these E2F proteins required for the physical association of pocket proteins is interdigitated with a potent trans activation domain (Helin et al., 1993; Flemington et al., 1993). It is likely therefore that the temporally regulated association of pocket proteins with DRTF1/E2F during cell cycle progression reflects its dynamically changing composition.
In many types of cells DP-1 is a frequent component of DRTF1/E2F, for example in 3T3 cells where it is present in DNA binding complexes which occur throughout the cell cycle (Bandara et al., 1994). Another member of the DP family, DP-2, is expressed in a tissue-restricted fashion (Girling et al., 1994). However, apart from this distinction the biochemical and functional properties of DP-1 and DP-2 are similar (Girling et al., 1994).
Recently, two publications have documented the existence of a further member of the DP family of proteins (Wu et al., 1995; Zhang and Chellappan, 1995). The cDNA sequence presented in Wu et al. (1995) is derived from the human DP-3 locus. Zhang and Chellappan (1995) report an identical sequence apart from the addition of a single glutamine at position 97.